A monomer containing unsaturated polyalkylene glycol (herein below, referred to as an “unsaturated polyalkylens glycol-based monomer”) is classified into an ether-based monomer having an ether bond (herein below, referred to as an “unsaturated polyalkylene glycol ether-based monomer”) and an ester-based monomer having an ester bond (herein below, referred to as an “unsaturated polyalkylene glycol ester-based monomer”). The polymer compound obtained by polymerization of an unsaturated polyalkylene glycol-based monomer is useful as a raw material for producing various polymers, and thus is used in various fields as cement admixture, a pigment dispersing agent, an anti-static agent, a resin, hardener, a dispersing agent for gypsum and water slurry, a dispersing agent for coal and water slurry (CWM), and a thickener or the like. For example, the polycarboxylic acid-based copolymer obtained by reaction between an unsaturated polyalkylene glycol ether-based monomer and an unsaturated carboxylic acid-based monomer is widely used as cement admixture, which dehydrates a cement composition by enhancing the fluidity of the composition (see, Patent Documents 1 to 3). Further, the unsaturated polyalkylene glycol-based monomer is advantageously used as an inorganic pigment dispersing agent for a pigment dispersion used for paper coating (Patent Document 4), or as a photo-sensitive resin composition for a photo-sensitive film or a photomask in a field of flat panel display, shadow mask for CRT, printed circuit board, and semiconductor or the like (Patent Documents 5 to 7).
When an unsaturated polyalkylene glycol-based monomer is used as a raw material for preparing a polymer, the corresponding monomer is generally used after it has been produced, transported, and/or stored. For such case, if the unsaturated polyalkylene glycol-based monomer is a solid at room temperature, the corresponding monomer is required to be dissolved by heating, and thus it is inconvenient in terms of handling for production. Further, since the unsaturated polyalkylene glycol-based monomer may undergo a polymerization reaction under heating and produce a gellified product, the heating process is not desirable.
Patent Document 8 discloses a method for storing and/or transporting a polyalkylene glycol ether-based monomer in the form of an aqueous solution. When this method is used, the heating process can be reduced, and thus the polymerisation caused, by heating is suppressed. According to the method, the base used for producing a monomer is not neutralized. However, for storage for a long period of time, it is preferable to perform neutralization. For such reasons, as a method including neutralization step, Patent Document 9 discloses a method for producing polyalkylene glycol ester including neutralisation step. It is disclosed in the document that, when (meth) acrylic acid polyalkylene glycol ester is added with phosphate, heated, and added with water to give an aqueous solution, stable and turbidity-free (meth) acrylic acid polyalkylene glycol ester with excellent color can be obtained.
According to this method, however, in addition to increased amount of a catalyst to be used, when the unsaturated polyalkylene glycol ether-based monomer is stored in aqueous solution state for a long period of time, turbidity or separation is caused, and thus insufficient stability is obtained.